1. Field of the Invention
The present invention relates to construction equipment and, more particularly, to a rotary tower crane having a tower consisting of a plurality of coaxial sections detachably connected to each other.
2. Description of the Prior Art
There are two main types of rotary tower cranes. One type has a needle-type boom of variable inclination. The other, and more common, type has a horizontal boom with a crab movably mounted on the boom to move in the lengthwise direction of the boom. The tower of either type of crane can be mounted on an undercarriage adapted to travel on rails, or else is mounted in fixed position on a base. The boom is pivotable with respect to the tower, either being pivotally mounted to the head of the tower and/or the tower head being rotatably supported about the longitudinal axis of the tower.
If rotary tower cranes are constructed with a freestanding tower, i.e., without external lateral support for the tower, the maximum height of the tower will depend upon the size of the cross section of the tower. The longer the freestanding tower is to be, the larger the cross section of the tower must be, to provide stability and strength to the structure. For example, the cross-sectional dimensions of the tower for tower heights above 30 meters are no longer determined by the lifting capacity of the rotary tower crane, but instead are determined by the stability required to withstand lateral wind pressure.
To obtain maximum benefit of a tower crane, it is desirable to have the tower crane as close to the work area as possible. Consequently, it is desirable that the tower of the crane have the smallest possible cross section, namely, the cross section sufficient only to handle the desired lifting capacity of the rotary tower crane. However, in order to assure stability of such a narrow tower crane against lateral wind pressure, it is necessary to anchor the narrow tower to the building alongside which the rotary tower crane has been placed for operation, at one or more places along the length of the tower.
Anchoring the tower to the building is disadvantageous, however, because the anchoring devices required, as well as their installation between the rotary tower crane and the building, are relatively costly. Therefore, it is considered preferable to make the towers freestanding, where practicable, in order to avoid having to use the anchoring procedures described above. Accordingly, relatively high towers have to be built from heavier sections having larger cross sections, while the towers of rotary tower cranes used only for small tower heights used tower sections of smaller cross section.
In view of the above, it becomes clear that at least two sets of tower sections of different cross-sectional size must be available and kept in stock. Even if this may not be necessary at the specific building site or the specific place of use of the rotary tower crane, nevertheless, such a supply of tower sections of different cross-sectional size must be kept available at a central storage place.